Piezoelectricity basically is a phenomenon of energy transformation between mechanical energy and electrical energy. As electric charges and strain have a cause-effect relationship, for material capable of generating piezoelectricity there is a crystal structure which can generate electron dipole moment when subject to an external force, a transformation phenomenon between mechanical energy and electrical energy takes places. This phenomenon is the source of piezoelectricity.
Generation of electron dipole moment is caused by a special arrangement between positive ions and negative ions in a crystal. Different crystal lattice systems in materials generate different electron dipole moments. The basic condition is the asymmetrical center of the crystal lattice. If there is a symmetrical center in the crystal lattice, the positive ions and negative ions are neutralized, then there is no electron dipole moment. And there is also no transformation between the mechanical energy and electrical energy. There are generally three types of transformation between the mechanical energy and electrical energy: 1. transforming mechanical energy to kinetic energy; 2. transforming electrical energy to mechanical energy; and 3. transforming electrical energy to mechanical energy, then to electrical energy again to be output. Piezoelectric ceramics have piezoelectric characteristic to perform mutual transformation between mechanical energy and electrical energy, thus are widely used in many types of applications, such as sensors to detect pressure, accelerometers, micromotors and the like. Moreover, piezoelectric films fit very well to the prevailing trend of slim and light of electronic products, and can be used on electronic elements for bio-detection and communication, thus receive a lot of attention in academics and industries.
The conventional piezoelectric structure adopts the principle of “unipoled PZT” discovered by Berlincourt in 1973. The principle is: alter the thickness relationship of the output/input area and the polarized area of a corresponding piezoelectric structure, a polarization process can be generated to change the step-up and step-down ratios of the piezoelectric structure. However, the conventional practice generally adopts a piezoelectric structure with a fixed distance between input/output electrodes and a ground electrode, and changeable polarized areas of the input/output electrodes to alter the step-up and step-down ratios. Such an approach creates a problem, namely to alter the step-up and step-down ratios, the polarized areas of input and output electrodes have to be changed, and the size of the piezoelectric structure also has to increase or decrease. This greatly affects the manufacturing techniques and installation space of the piezoelectric structure in the applications of industries.
U.S. Pat. No. 6,707,235 discloses an annular piezoelectric structure which includes a first portion and a second portion that generate vibration through an AC power supply and other portions to transform the piezoelectric structure. Its polarized direction is vertical to the perimeter surface of the annular body. It uses different shapes of the piezoelectric structure to generate different step-up and step-down ratios. The different shapes cause fabrication difficulty. R.O.C. patent No. I253772 discloses a piezoelectric structure to provide an improvement. It has an output electrode and an input electrode spaced from a ground electrode at different distances. By changing the distance of the input electrode or output electrode relative to the ground electrode the step-up and step-down ratios of the piezoelectric structure can be altered. But its polarized direction is limited to the thickness direction, and the output/input voltage is determined by different thickness of the piezoelectric structure of the output/input electrodes. To get a greater output/input voltage ratio, the thickness difference also is greater. Hence the heights of the inner and outer rings of the circular piezoelectric sheets are uneven. Moreover, when the thickness is excessive it takes too much space of the already constrained internal space of electronic equipments. Hence how to improve the piezoelectric structure is a big issue remained to be overcome in the industry.